Insert Face equally from all sides
I think the main culprit behind the trouble you are having is because of non-uniform scale. What I mean by this, is that every time you scale an object in Object Mode, you should apply scale (Ctrl + A &gt; Scale), or it can and will cause a huge raft of issues later, from problems with texturing, to insets, to issues with curves and arrays.
Take this example - All I did was make a scene with 2 default cubes. One of the cubes (the one on the left) I scaled by 2 on the Z-axis in edit mode, and the other (the one on the right),  I scaled by the same amount in Object Mode. I then entered Edit Mode, and inset the front face of each of them by the same amount. I also added a horizontal and vertical loop cut around each so the &quot;Edge Width&quot; overlay will give more useful measurements.

Notice the difference between the two - not just as to the shape of the inset, but also as to how &quot;long&quot; blender&quot; thinks each of the edges are. (ie: the cube scaled in Object Mode &quot;thinks&quot; it's still the original 2M long on the Z-axis).